Hello, I have been teaching Logo then Scratch for years but am a bit dumbfounded on how to change games to ipad use where there are no key press triggers available to students.. What triggers can be used? Any ideas?

The <mouse down?> block responds to the screen being pressed on touchscreen devices. You can try making arrow key sprites and using them in conjunction with <mouse down?> to replace the need for a keyboard. It's a bit inconvenient, but it works.

You can put on-screen control buttons. Do this by putting sprites that have a “when this sprite is clicked” code.
There are examples on the upper right corner of this starter project that I give to my students.
https://scratch.mit.edu/projects/541229388

best wishes,
Mike Roam